In processes for high-volume production of glass articles such as plate glass, the glass material is heated to produce molten glass, and the molten glass so produced is molded to manufacture glass articles such as plate glass. If the molten glass is heterogeneous, striae may form in the glass articles. Striae are zones of striated appearance differing in refractive index and/or specific gravity from the surrounding area; in optical components such as lenses, substrates for liquid crystal displays (LCD), and other such applications, there is a need to rigorously eliminate them from the glass articles. Particularly in the case of glass substrates for displays, of which substrates for liquid crystal displays are representative, it is necessary to keep striae to extremely low levels throughout the entire large area of the surface. Stirring of the molten glass by a stirring device is performed in order to prevent striae. A typical stirring device is provided with a chamber of round cylindrical shape, and a stirrer. The stirrer has a shaft serving as a rotation axis, and blades connected to the side wall of the shaft. The molten glass is introduced into the chamber in which the stirrer is disposed, and the molten glass is stirred by the blades, rendering the molten glass homogeneous.
Patent Document 1 (Japanese Unexamined Patent Application Publication S63-8226) discloses a stirrer having a shaft serving as a rotation axis, and blades connected to the side wall of the shaft. The blades of the stirrer disclosed in Patent Document 1 are inclined with respect to the stirrer axis, and thereby create a flow of the molten glass towards the upward direction or downward direction of the chamber, enhancing the stirring effect.
Patent Document 2 (Japanese Unexamined Patent Application Publication S58-88126) likewise discloses a stirrer having a shaft serving as a rotation axis, and blades connected to the shaft so that the faces thereof are inclined in the circumferential direction (rotation direction) of the shaft. This shaft is provided with several tiers of blades in the lengthwise direction of the shaft, with blades of adjacent tiers inclined in opposite directions. Between the blades of adjacent tiers, there arise two glass flows, i.e., glass flow arising from downward pushing of the molten glass, and glass flow arising from upward pushing of the molten glass, and these flows collide. It is conceivable that higher stirring effect can be obtained, as compared with a case in which the molten glass flows in one direction only.